1. Field of the Invention
This invention relates to a circuit for controlling the power applied to the drive motor of an electric vehicle from the battery of the type employing a semi-conductor switch and duty cycle control for the switch.
2. Prior Art
The development in recent years of high power, relatively low cost semi-conductor switches has led to their frequent employment in switching, duty cycle adjustment, speed control systems for direct current motors. The switches are disposed in series with a DC power source and the motor and the "on" time of a multi-vibrator controls the closure of the switches and according the power applied to the motor.
Controlled rectifiers are used as the switching element in many power control systems and transistors, which have the virtue of eliminating the need for complicated turnoff circuits in DC applications are employed in others. Typically, the semi-conductor switches used in these motor control units are controlled by the output of a multi-vibrator. The multi-vibrators are fed by an oscillator and controlled by a signal derived from the accelerator so as to vary the ratio of "on" time to "off" time during the period between two oscillator cycles. Typically, the output of these multi-vibrators may be adjusted continuously between two extremes: at one extreme the multi-vibrator is at full "off" and no power is delivered to the motor from the batteries through the switches; at the other extreme the multi-vibrator is only in the "off" state for a small percentage of each switching period. A conventional multi-vibrator is incapable of shifting its state between 100% "on" and 100% "off".
When controlled rectifiers are used as the semi-conductor switch, the mandatory "off" portion of each clock cycle is increased by the time required to recharge the turn-off capacitor. As a result of this "rest period" in each clock cycle, full battery power is never delivered to the motor and the battery must be designed to have a greater power delivery capacity than would be the case if full battery power could be delivered to the load.
One of the objects of the present invention is to use transistors as semi-conducting switching elements in order to eliminate the need for the non-conductive time associated with recharge of the turn-off capacitor and to use a unique multi-vibrator to power the switch which is capable of providing control over a full range from continuously "off" to continously "on".
In addition to the speed or power control switching provide to the accelerator and other appropriate speed control, such DC motor drive circuitry must be provided with over-current protection for the motor. With convention controlled rectifier systems over-current protection has taken the form of means for sensing the current through the motor and firing the turn-off capacitor if the motor current exceeds a predetermined value. Because of the need to recharge the turn-off capacitor, the turn-off is typically continued through the balance of the clock period, often resulting in a series of sharp current surges through the motor when an excessive current occurs.